The present invention relates to image block matching methods and in particular to a method of matching blocks of pixels between two images which operates in the domain of an orthogonal transform.
Motion estimation a fundamental type of digital video processing. Motion estimation can be found in many digital video applications, for example, video coding, motion compensated video standards conversion and machine vision. Motion information in the context of video coding is useful to reduce signal entropy so that a compressed video signal can be transmitted through a bandwidth-constrained channel. Motion information in the context of video standards conversion helps the converter to recover missing data, spatially and/or temporally, by interpolating the signal along the motion path. Motion information is useful in machine vision applications to allow a robot to find or follow a workpiece through various processing steps. In each of these instances, motion estimation typically uses a pixel-domain block matching (BM) technique and the matching criterion is the minimum mean-square-error (MSE) between the motion block in the current frame and the trial matching blocks in the search frame. The minimum MSE may be replaced by minimum mean absolution difference (MAD) for a more cost-effective hardware implementation.
Pixel-domain BM techniques are well suited to these applications because of their algorithmic simplicity and because they may be implemented using very large scale integrated (VLSI) circuitry. These existing BM techniques may provide less than optimal results, however, because they are based on pixel-domain comparison and because the best match is determined based on the minimum error index (MAD/MSE). It is generally agreed that the pixel-domain MSE (or cross-correlation) measure is not fully consistent with image visual error as perceived by humans. Consequently the error measure currently used in motion estimation requires further modification. In other words, the characteristics of human visual system should be considered during the matching process.
This invention is embodied in a motion estimation technique that takes human visual characteristics into consideration. The scheme uses transform-domain block matching techniques to achieve more accurate motion vectors than can be achieved using pixel-domain block matching techniques.
According to one aspect of the invention the motion estimation method performs a transform operation on the motion block and on all trial matching blocks. The method then compares the transformed motion block to each of the transformed trial matching blocks and selects, as the matching block, the trial matching block which exhibits the smallest difference with the motion block in the transform domain.
According to another aspect of the invention, the transform operation is a Hadamard transform operation.
According to another aspect of the invention, the method calculates the mean-squared error (MSE) between the transformed motion block and each of the transformed trial matching blocks and selects, as the matching block, the transformed trial matching block having the minimum MSE.
According to another aspect of the invention, the method calculates the mean absolute difference (MAD) between the transformed motion block and each of the transformed trial matching blocks and selects, as the matching block, the transformed trial matching block having the minimum MAD.
According to yet another aspect of the invention, the method allows the size of the motion block to be changed.